Preschool-age children are one of the three high risk groups for morbidity attributable to soil-transmitted helminth infections (STH). At this age, the STH infections of most importance are Ascaris and Trichuris infections. WHO recommends deworming of preschool-age children as of 12 months of age, but there is no empirical evidence on what deworming exposure (in terms of age at administration adn frequency of administration) has the most beneficial impact on health. This research project therefore proposes to follow a cohort of preschool-age children who are known to have had different deworming exposure histones between 12 and 24 months of age. By following these children up to five years of age, cumulative health benefits will be measured and the optimal deworming strategy identified. It is expected that these health benefits will extend into later childhood and beyond. Research results will also be used to inform deworming health policy and strategies in STH-endemic regions globally. RELEVANCE (See instructions): An important window of opportunity exists between 12 and 24 months of age when health and nutrition interventions play a vital role in both short and long term health. It is important to measure this health impact, not only because it fills a research gap, but also because it is useful feedback for program managers, and other partners. In following children up to five years of age, this cohort study will provide key Rmf^irica! ftvidenr.fi on the health impact of different dewnrmina strateaifis tarafited to this high ri.sk ana